The coating of electrically conductive substrates by electrodeposition is a well known and important industrial process. (For instance, electrodeposition is widely used in the automotive industry to apply primers to automotive substrates). In this process, a conductive article is immersed as one electrode in a coating composition made from an aqueous emulsion of film-forming polymer. An electric current is passed between the article and a counter-electrode in electrical contact with the aqueous emulsion, until a desired coating is produced on the article. The article to be coated is the cathode in the electrical circuit with the counter-electrode being the anode.
Resin compositions used in cathodic electrodeposition baths are also well known in the art. These resins are typically manufactured from polyepoxide resins which have been chain extended and adducted to include a nitrogen. The nitrogen is typically introduced through reaction with an amine compound. Typically these resins are blended with a crosslinking agent and then salted with an acid to form a water emulsion which is usually referred to as a principal emulsion.
The principal emulsion is combined with a pigment paste, coalescent solvents, water, and other additives at the coating site to form the electrodeposition bath. The electrodeposition bath is placed in an insulated tank containing the anode. The article to be coated is made the cathode and is passed through the tank containing the electrodeposition bath. The thickness of the coating is a function of the bath characteristics, the electrical operating characteristics, the immersion time, and so forth.
The coated object is removed from the bath after a set amount of time. The object is rinsed with deionized water and the coating is cured typically in an oven at sufficient temperature to produce crosslinking.
The prior art of cathodic electrodepositable resin compositions, coating baths, and cathodic electrodeposition processes are disclosed in U.S. Pat. Nos. 3,922,253; 4,419,467; 4,137,140; and 4,468,307 which are incorporated herein by reference.
Cathodic electrodeposition is widely used in the automotive industry because it gives superior corrosion protection, it covers recessed or hard to reach areas, it deposits a uniform film thickness free of voids and defects such as sags or runs, it is not labor intensive, it has less emissions to the environment and there is less exposure to workers.
Typical commercial cathodic electrocoat films are not conductive after cure. However, having a conductive electrocoated film can be advantageous for a number of reasons. For instance, a conductive film allows re-electrocoating over the initial electrocoat layer. Re-electrocoating can be beneficial since current commercial cathodic electrocoat systems have certain practical limits to the amount of film build obtainable, and during the cure there is a tendency for the film to pull away from sharp edges thereby reducing edge corrosion protection. One way of increasing the film build and improving edge coverage is by re-electrocoating over the initial cured film. But in order to re-electrocoat it is necessary that the initial coat have sufficient conductivity to allow the re-electrocoat process to occur. (Re-electrocoating over a non-cured non-conductive electrocoat film is possible but it is not a preferred method. This "wet-on-wet" re-electrocoating does not solve the edge coverage problem.)
A conductive film might also be beneficial since it could improve transfer efficiency of electrostatic spraying over the electrocoated film. This is an advantage because less paint is used to get the same coverage and it would also minimize air pollution.
Applying conductive electrocoated films has been done in the past by using conductive powders in the electrocoat composition. Conductive powders currently used to make the electrocoat layer conductive are carbon black, graphite or noble metal powders. The noble metal powders are not usable in cathodic electrocoat because their density would cause them to settle in the bath and on horizontal surfaces of the object being electrocoated. Carbon black and graphite have the limitation that they are black in color and therefore can only be used in black or very dark colors. This is a disadvantage because the dark color requires the use of a thicker layer of topcoat to cover the darker color of the electrocoat primer.
A further disadvantage of using carbon black or graphite powders is that they can only be used at relatively low pigment concentrations (5/100 or less pigment to binder ratio) because at higher concentrations film appearance becomes unacceptable (e.g. orange peel). The problem with the low pigment concentrations necessary when using carbon blacks is that they result in poor edge coverage because of excessive flow out from sharp edges during baking.
What is needed is a cathodic electrocoat composition which forms a conductive film which is light in color and allows higher pigment to binder ratios to improve edge coverage.